In a general cellular network, communication is performed through a direct link between a base station and a terminal within a cell covered by the base station. However, a communication network tends to increasingly have a higher frequency band and radius of cells tend to reduced to accommodate high speed communication and increasing traffic. Thus, operating a cellular wireless network scheme as it is through a direct link involves a great deal of problems.
Thus, in order to solve such problems, a relay system which is distributedly controlled and established and able to actively cope with a change in an environment such as addition of a new base station has been proposed in a next-generation communication system.
A relay system extends cell service coverage by covering a partial shadow area in a cell region, increase a system capacity, and reduce a burden of initial installation cost by using relaying in an initial situation in which there is not many service requests.
FIG. 1 shows a conventional relay system.
As shown in FIG. 1, one or more terminals 11, 12, 13 exist in a region of a base station 20.
In this case, one or more of the terminals 11, 12, 13 may operate as a relay (or relaying). A terminal operating as a relay has a good channel environment and a high channel gain on average.
Namely, when the terminal that has a good channel environment operates as a relay, the other terminals that do not have such a good channel environment may transmit and receive data to and from the base station through the terminal operating as a relay.
The method of utilizing a terminal as a relay in a cellular system can improve transmission performance. Namely, the terminal, which serves as a relay to allow the other terminals existing within the base station coverage to stably communicate with the base station, amplifies a data signal and transfers the amplified data signal to each reception end, thus improving transmission performance.
Meanwhile, there are two types of schemes for each relay to relay a signal. Namely, an amplify and forward (AF) scheme and a decode and forward (DF) scheme exist.
In the case of the DF scheme, a relay decodes a signal received from a transmitter, re-encodes it to generate a message signal, and transmits the message signal to the receiver. However, in the DF scheme, since the relay decodes the message signal of the transmitter, re-encodes the message signal, and then, transmits the signal, noise influence is less made but hardware complexity is increased due to decoding of the message signal of the transmitter.
In the AF scheme, the relay amplifies a message signal received from a transmitter to have a predetermined size and re-transmits the amplified signal. The AF scheme does not perform signal processing for decoding, simple hardware can be implemented. However, since noise added in the relay is transferred to a receiver, performance is degraded in an environment having a low signal-to-noise ratio.
Compared with the DF scheme, the AF scheme advantageously has low complexity and processing delay in an actual implementation, and thus, recently, the AF scheme is more actively studied.
However, according to the AF scheme as mentioned above, in order to decode the signal received from the relay station, the receiver should estimate a channel between the receiver and the relay and also should know a channel between the transmitter and the relay.
Meanwhile, a relay scheme of the relay station includes a two-way relay scheme and a one-way relay scheme.
The two-way relay scheme allows the relay to perform two-way communication with the base station and the terminal simultaneously, so it can remarkably improve a channel capacity in comparison to the one-way relay scheme.
When the two-way relay technique is applied in an SISO (Single-Input Single-Output) manner to the relay, the relay may only need to simply amplify and forward (AF) the received signal.
However, unlike the SISO scheme, when the two-way relay scheme based on MIMO (Multiple Input Multiple Output) is applied to the two-way relay scheme, the relay station can enhance spectrum efficiency relay data at a high speed by using a plurality of antennas.
In the MIMO type two-way relay scheme, the relay should filter received multiple signals to configure multiple outputs, so filter design of the relay station acts as an important factor of system performance. In other words, DF (decode and forward) should be performed, and in this case, filter design works as an important factor of a system.
Also, the base station and the terminal require a storage medium capable of storing a signal transmitted by them, respectively, and when a signal, which is transmitted by the base station and the terminal, rather than a target reception signal is returned in a channel, the signal acts as a self-interference signal. Thus, accurate information regarding a channel associated with the self-interference signal is required and a function of removing self-interference based on the information is required.
However, in the case of the terminal, due to a hardware limitation such as battery performance and size, the terminal may not have a self-interference removal function, and also, in this case, a filter design of the relay station is further important.
When the AF scheme of the conventional art as described above is used, the receiver should know a channel between the transmitter and the relay, as well as a channel of the relay, but this accompanies a great amount of technical difficulties in actuality.
Thus, in order to enable the receiver to estimate a channel between the transmitter and the relay, a scheme of allowing the transmitter and the relay to use a subcarrier and a pilot signal, respectively, has been proposed.
However, the scheme of allowing the transmitter and the relay to use a subcarrier and a pilot signal, respectively, is greatly ineffective in terms of channel and causes a waste of radio resource.